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I am being a bit slow about this problem but I have not quit thinking of solutions. The best thing I can still come up with is to use a ACS712 bi-directional current sensor. That should be able to read even the inductive spikes and give a clean signal to the mpguino that will be able to remain as is with no modifications.
My plan is a small board with a 7805, bridge rectifier, ACS712, and a 741 op amp. The ACS712 will go inline with the rectfied injector wire with the 7805 powering it. The output will use the opamp to sense voltage over 2.55 or so and go high at 12V and feed that signal into the mpguino that will then cut it back to 5v and should reliably work.
Optionally instead of using a bridge rectifier to keep the output voltage of that current sensor between 2.5V and 3V a dual opamp could be used that is set as a window switch to turn on above 2.55V and below 2.45V but that would leave a little notch in the signal that would probably mean a missed interrupt like I am getting with the simple opamp circuit I tried last.
If the rectified signal works good then I say leave it just for the extra reliability. It will be needed to keep the current flowing one direction through the current sensor and make the inductive spike much smoother looking to the chip and give better readings hopefully.
I should have a few ACS712 chips here tomorrow and will try and build the circuit this week and see what I can figure out.
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